I think IMO the 90s era of Sentai is the best. So far we have these trademarks that went on although as usual NOT in all shows:
This was the most formally dressed era.
It was a time for more unique storytelling methods. Because of this, even mecha started to change in itself like the introduction of the artificial intelligence mecha. Jetman is the most notable and second is Dairanger. My favorite is still Jetman. :-P
There has been a better blend of humor and seriousness as to make it more like the real life and the 90s was a good start.
I consider this era to be when ladies will be interested with Super Sentai because of better-looking male roles.
There was also plenty of action. I consider the 90s to be the best with fighting scenes.
In Zyuranger, the trademark sixth ranger was soon added thereafter. Some of them existed as extra heroes who were not officially rangers but were more of allies which is still continued to today. From the extra ranger also came the color silver in Megaranger.
This era had the unforgettable moment of a kid ranger in Dairanger and later in Ohranger. I guess that's why Saban created Justin for Power Rangers Turbo.
It kind of got Ultrazord-heavy ever since the 80s era ended with Turboranger. It was used in Fiveman, Zyuranger, Dairanger and Ohranger.
The introduction of a self-operating robot.
The introduction of a secondary combining robot that used some other means to finish the enemy than the trademark sword. It was first used in Kakuranger.
The introduction of a combining robot from five combining robots although unlike in the Transformers cartoon, these robots didn't become a much bigger combination.
Also there had been more unique than the stereotypical Sentai villains. Jetman's Vyram was for an example of such. In fact, villain falls were also made more creative like how Radiguet ironically fell because of the wound on his back or how Exhaus was defeated by expired imo-youkan which added ironic twists to how such mighty badass villains fell to such simple attacks in the end.